The adoption of mobile TV has been generally frustrated by the business model and the cost of building new infrastructure to support mobile TV reception. As a result the adoption of new mobile TV standards such as DVB-H has been much slower than anticipated and only the Japanese ISDB-T 1-seg and Korean T-DMB standards have enjoyed any level of commercial success.
In contrast the adoption of terrestrial digital TV DVB-T has been rapid in Europe driven by legislation and the reallocation of broadcast spectrum. The present invention seeks to take advantage of terrestrial digital TV broadcast infrastructure by allowing existing free-to-air terrestrial digital TV broadcast signals intended primarily for TV reception in the home, to be received and displayed on a mobile phone and other handheld devices.
The main disadvantage with the reception of such signals is that the resolution of displays in the home is much higher than that which can be displayed on a mobile device. Processing and displaying such broadcasts therefore involves adapting the broadcast signal so that it can be displayed on a target display with much lower resolution, potentially ¼ or less resolution compared with a typical TV receiver present in the home.
One of well-known limitations in the design of a block-based video decoder for a portable device, such as mobile phone, is to support a high input resolution and at the same time providing high quality video decoding on the low resolution output. If typical design assumes that the decoded reference frames are saved in external memory (commonly SDRAM) in high resolution, unavoidable frequent memory access results also in high power consumption which is inappropriate for the mobile decoder.
It is possible to compress the reference frames after reconstruction and store them in the buffer for subsequent use. To operate efficiently, such a video coding system would require efficient and low complexity cost frame compressor and de-compressor. However, when developing an image compression system with random data access targeted to achieve a high compression rate, one obviously has to employ lossy compression method. This is not always possible for video coding system, where decoded frames have strict temporal dependency on each other. Current reference frame compression systems do not contain special provisions to cater for high resolution input followed by low resolution output.
Herein, the most advantageous approach would be to downsample video as early as possible during the decoding process. Thus, the Motion Compensation block of such a video coding system has to deal with frames at the low target output resolution, rather than the high input resolution. This solution has the advantage of having the memory requirements derived from the targeted low resolution output, not from the high resolution input. Also, high resolution video contains much detail that is redundant for low resolution display.
A number of approaches have been proposed for the MPEG-like decoders which can be divided into two categories, depending on where the downsampling is performed in the decoding process. In particular, the downsampling can be performed on the data being still in the compressed domain (i.e. IDCT), which provides additional complexity reduction, such as U.S. Pat. No. 5,708,732. However, these suffer from the disadvantage of being IDCT-domain specific and having fixed downsampling factor, i.e. explicitly designed for, say, MPEG-2 8×8 IDCT and, thus, cannot be utilized by other video decoders or programmed for arbitrary downsampling factors.
The second category assumes the actual downsampling prior the Motion Compensation process, i.e. after passing unmodified IDCT. In that case, there is wide choice of downsampling algorithms available ranging from low to high computational complexity, but adopting any of them leads to appearance of visible video decoding artifacts due to a nature of the downsampling method.